Recently, a great deal of interest has been increasingly directed to energy storage technology. In particular, applicable fields of such energy storage technology have been extended to power sources for portable telecommunication instruments such as mobile phones, camcorders and notebook computers, and further to power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs). As such, efforts and attempts to research and develop batteries capable of implementing such technology and power sources are increasingly undertaken. In this respect, the field of electrochemical devices has been receiving a great deal of attention, and in particular, a lot of interest has been focused on development of rechargeable secondary cells. In accordance with the trend towards development of such batteries, research and development has been focused on design of a new type of battery and electrode which increases charge density and specific energy.
Among currently applied secondary batteries, lithium ion batteries, developed in the early 1990s, have received a great deal of attention due to their high operation voltage and energy density as compared to traditional batteries using aqueous electrolytes, such as Ni-MH, Ni—Cd and PbSO4 batteries. However, such lithium ion batteries suffer from safety problems associated with flammability and explosiveness, due to use of organic electrolytes, and difficult and complicated manufacturing processes. State-of-the art lithium ion polymer batteries have received a great deal of interest as a next generation battery in which drawbacks exhibited by such lithium ion batteries have been alleviated. However, current lithium ion polymer batteries still have a lower charge capacity than existing lithium ion batteries, and in particular have insufficient discharge capacity at low temperatures, thus urgently requiring improvement in such poor discharge capacity.
The operation mechanism of lithium ion batteries is different than that of conventional batteries. LiCoO2 and graphite, utilized as cathode and anode active materials in lithium ion batteries, respectively, have crystalline structures with cavities therein. Upon charging and discharging the battery, lithium ions migrate inside the battery by entrance and exit of lithium ions into and from the cavities.
The cathode of the battery is a current collector serving to collect electrons and aluminum foil is generally used as the cathode. The active material, LiCoO2 is coated on the aluminum foil. However, LiCoO2 exhibits low electron conductivity and thus carbon is added in order to enhance electron conductivity.
The anode is copper foil coated with graphite, as a current collector. Graphite has superior electron conductivity and generally, electron conductive material is thus not added to the anode.
The anode and cathode are isolated from one another by a separator, and as the electrolyte, liquid prepared by addition of lithium salts to the organic solvent is employed.
Secondary batteries are prepared in a discharged state. Upon charging, lithium ions present in LiCoO2 crystals exit and migrate to the anode and then enter into graphite crystal structures. In contrast, upon discharging, lithium ions in graphite exit and enter crystal structures of the cathode. In this manner, as charging and discharging of the battery proceeds, lithium ions alternate between the anode and cathode, the phenomenon of which is called “rocking chair concept”, which corresponds to the operation mechanism of the lithium ion batteries.
Numerous manufacturers produce such batteries but the safety characteristics of the produced batteries vary from one manufacturer to the next. However, evaluation of safety of such batteries and safety securing are very important. The most important consideration is the requirement that the battery must not cause injury to users upon error and malfunction in operation thereof. For this purpose, safety standards strictly regulate fire ignition and fuming or smoking in the battery.
A variety of methods have been conceived to effect safety improvement. In this connection, there has been filed a patent application relating to a technique of fabricating a battery using more than two types of separators. Japanese Patent Publication Laid-open No. Hei 10-199502 discloses a battery having both high tensile strength and high capacity retention properties by stacking two separators having different characteristics between the cathode and anode. In this patent, the first and second separators are based on a polyolefin resin and polyamide resin, respectively.
Japanese Patent Publication Laid-open No. 2000-82497, assigned to Sony Corporation, employs two identical separators that were wound each other, in order to improve cycle characteristics of the battery, but this exhibited battery characteristics irrespective of safety thereof.
Japanese Patent Publication Laid-open No. 2003-243037, assigned to Shin-Kobe Electric Machinery Co., Ltd., discloses a lithium ion battery having improved safety by using two separators having different melting points. Herein, the safety of the battery is improved by inducing primary short-circuiting, when the temperature of the battery elevates, in a second electrode zone that does not occlude/release lithium ions and is composed of the second separator having a lower melting point. However, in this case, the practical range in which the battery can be operated is limited to about 90° C., and thereby, when short-circuiting occurs at below such a temperature range, severe deterioration in battery performance occurs, thus primary short-circuiting is required to occur over 90° C. However, where internal short-circuiting occurs at temperatures higher than 90° C., the practical battery may be exposed to more dangerous situation compared to occurrence of short-circuiting at room temperature, which in turn probably leads to worsening safety of the battery. As a result, this method cannot be a good solution. In addition, use of polymer separators having different melting points considers only elevation of the battery temperature, and has no effects on the battery safety when short-circuiting occurs by external impact such as crushing, partial crushing or the like.
As such, there remains an urgent need in the art for development of an electrochemical device for improving the safety of batteries, upon application of external impact such as crushing, partial crushing or the like.